Why bother with a mandrel for the interior?

Lookiong at the illustration of how the pieces are aligned it seems rather obvious that the interior sides of each strip stay flat during assembly, and are not machined in any way.

So, why not just cover each strip on one side with a layer of thin glass cloth and resin? You can cut the notches either before or after you put on this glass layer.

If you simply lay all your strips side by side , with about a 1/4 inch space between them, you can coat them all together with a layer of resin using a roller. Then just drop a 6 inch wide piece of glass cloth over all the resin-coated strips and wipe over it with a squeegee to tack the cloth in place. (You might want to think of doing this at a time when you are glassing a boat and have a bit of resin left in the tray, or the roller is still wet.)

After the resin sets up you can cut between the strips with a utility knife. trim close to the square edge of a strip, leaving a little bit of excess glass cloth hanging over the V cut edge of each strip.

If you look at a sideview of each of these strips it will look like each notched edge has a tail of glass cloth hanging out. Go back to your original illustration and imagine how all those extra tails would overlap the adjacent strips on the interior of the paddle shaft.

When you go to glue up your paddle shaft the glassed side of each strip is on the interior of the shaft. apply your resin to the notch, and let a bit more go onto the overhanging glass fabric. The excess glass fabric should be 'springy' enough to make contact with the adjoining piece, bridging the joint, and adding a bit of extra strength to each joint.

You could probably use a light glass cloth for this (say 1 to 3 ounce), or maybe go as high as 6 ounce fabric. The thicker the cloth the more resin it will soak up, and the more the paddle will weigh. Scraps can easily be pieced together for this job. Bad seams won't show.

Just a thought

PGJ